1. Field of the Invention
This invention relates to a radiation image read-out method for reading out a radiation image stored on a stimulable phosphor sheet by exposing the stimulable phosphor sheet to stimulating rays which cause it to emit light in proportion to the stored radiation energy, and photoelectrically detecting the emitted light, and to an apparatus for carrying out the method. This invention also relates to a radiation image read-out and reproducing method for reading out a radiation image stored on a stimulable phosphor sheet to obtain an image signal and reproducing a visible image on a photosensitive material by use of the image signal, and to an apparatus for carrying out the radiation image read-out and reproducing method.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object such as the human body to have a radiation image of the object stored thereon, and is then exposed to a stimulating ray beam such as a laser beam which causes the stimulable phosphor sheet to emit light in proportion to the stored radiation energy. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, and the radiation image of the object is reproduced as a visible image by use of the image signal on a recording medium such as a photographic film, a display device such as a cathode ray tube (CRT), or the like.
The radiation image recording and reproducing system using a stimulable phosphor sheet is advantageous over conventional radiography using a silver halide photographic material in that the image can be recorded over a very wide range of radiation exposure. More specifically, since the amount of light emitted upon stimulation after the radiation energy is stored on the stimulable phosphor sheet varies over a wide range in proportion to the amount of said stored energy, it is possible to obtain an image having a desirable density regardless of the amount of exposure of the stimulable phosphor sheet to the radiation, by reading out the emitted light with an appropriate read-out gain and converting it into an electric signal to reproduce a visible image on a recording medium or a display device.
Therefore, in the aforesaid radiation image recording and reproducing system, high sensitivity image recording may be conducted by adjusting the radiation dose to a very low value.
However, in the case where a radiation image is recorded on the stimulable phosphor sheet with a low radiation dose, only a low level of radiation energy is stored on the stimulable phosphor sheet, and the level of light emission by the stimulable phosphor sheet in proportion to the stored radiation energy becomes generally low. As a result, graininess of the visible radiation image reproduced based on the read-out image signal deteriorates (i.e. becomes more perceptible). Consequently, the reproduced visible image becomes rough, and the image quality thereof becomes low.